Shocking levels of cancer-causing chromium-6 have been found in water supplies across the US

About two-thirds of Americans are exposed to unsafe levels of a cancer-causing chemical compound in their drinking water, according to a new analysis.

The Environmental Working Group tested more than 60,000 water samples between 2013 and 2015 and found chromium-6 in more than 75 percent of the samples, six years after its analysis found elevated levels of the compound in 31 of 35 cities sampled.

The Oklahoma City and Phoenix areas had by far the highest levels of contamination, followed by St. Louis County, Houston, Los Angeles and Suffolk County, New York.

Water in Cleveland County, just south of Oklahoma City, found up chromium-6 levels at an average of 29.59 parts per billion -- with a high reading of 97.38 ppb -- which is exponentially higher than the California Public Health Goal of 0.02 ppb.

See the chromium-6 levels in your county with this interactive map.

The study found that every U.S. water system that serves more than 1 million people has potentially harmful levels of chromium-6.

The movie "Erin Brockovich" depicted the real-life story a lawsuit filed by residents of Hinkley, California, against Pacific Gas and Electric Company after dangerous levels of the carcinogenic chromium-6 were found in their drinking water.

"More than 20 years ago, we learned that this dangerous chemical poisoned the tap water of California communities, and now these tests and EWG’s report show that roughly 218 million Americans are being served drinking water polluted with potentially dangerous levels of this known carcinogen,” Brockovich said.

“But in that time the EPA hasn't set drinking water standards for any previously unregulated contaminant, and there are disturbing signs the agency may again do nothing about chromium-6,” she added. “This is an abject failure by the EPA, including members of Congress charged with overseeing the agency, and every American should be outraged by this inaction.”

Only California has a legal limit for chromium-6 -- which is used in steel production and in electrical plants' cooling towers -- in drinking water, and Brockovich said even that is set too low.

Airborne chromium-6 particles can cause lung cancer, and drinking contaminated water can cause stomach cancer, liver damage, reproductive problems and harm to children's brain development.